1. Technical Field
The present invention relates to a power assisted steering system for a vehicle. In particular, the present invention relates to a power assisted steering system which varies the amount of power assist as a function of vehicle speed.
2. Description of the Prior Art
U.S. Pat. No. 4,561,516 discloses a power assisted vehicle steering system which varies the amount of power assist as a function of vehicle speed. The steering system includes a hydraulic control valve for directing fluid under pressure to a power assist motor. The control valve includes a valve core rotatable within a valve sleeve. The valve core is formed as an integral part of the steering input shaft. The outer surface of the valve core and the inner surface of the valve sleeve have cooperating lands and grooves. Relative rotation between the valve core and the valve sleeve varies the width and the area of orifices defined by adjacent edges of opposing grooves and lands, thus controlling the fluid flow through the valve in response to turning of the input shaft.
The valve sleeve shown in U.S. Pat. No. 4,561,516 is axially slidable relative to the valve core. Changes in vehicle speed cause a control valve in the fluid return line to progressively vary the pressure of fluid which bears on an end surface of the valve sleeve. Such pressure change causes the valve sleeve to move axially. As the valve sleeve moves axially in response to higher vehicle speeds, it unmasks relatively wide end portions of grooves on the valve core. As the relatively wide end portions are unmasked, the amount of power assist is reduced. Thus, as vehicle speed increases, the amount of power assist decreases.
The apparatus shown in U.S. Pat. No. 4,561,516 utilizes axial movement of the valve sleeve to vary the amount of power assist. The valve sleeve slides axially along the inside surface or bore of the valve housing. However, the area between the valve sleeve and the valve housing must be sealed against fluid leakage therebetween. This requires the valve sleeve to include relatively hard seals which bear against the softer surface of the valve housing, and significant valve bore wear can occur. As the valve bore wears, sticking movements of the valve sleeve can occur, resulting in unpredictable changes of steering assist levels. Eventually, axial displacement of the sleeve can be prevented and complete failure of the system results. It is therefore desirable to provide for relative axial motion between the two valve elements, without having the valve sleeve slide axially against the valve housing.